When my pay monthly contract with O2 was due to end in July 2014, a supplier phoned me to offer an upgrade to my current O2 phone. I agreed to look at a phone it suggested but, not liking it, returned it to the shop it had come from. I obtained a receipt and confirmation of disconnection.

I told O2 I was leaving, acquired the appropriate code and moved to another company. O2 initially said my contract was due to expire in June 2016 but then confirmed it had been cancelled within the cooling-off period. Nevertheless, in March 2015 I received a letter from a debt collection agency concerning a balance of £379 owed to O2 as a penalty for transferring early out of a new contract. I immediately phoned O2 and sent the paperwork it asked for. It said this would be sorted out.

John Harrop, West Yorks

Six months later you discovered that an O2 default was shown on your credit files. You had never had an adverse credit report before and contacted O2 several times. This got you nowhere.

Meanwhile, one of the credit reference agencies asked O2 to amend the entry. O2 said it wouldn’t.

You protested to the company and were assured the matter would be considered by a manager. Three weeks later, having heard nothing more, you wrote and were told it would be seen to. It wasn’t and you wrote to me.

O2 now says that the outstanding payment came about because the third party you had acquired the upgraded phone from had not informed it that you had returned the phone within the cooling-off period. Then, when you confirmed the facts to O2, the credit was not processed as it should have been and continued to show as an overdue payment.

O2 has now cancelled it and recalled the account from the debt collection agency. It says: “ We have cleared any marks on Mr Harrop’s credit files relating to this issue and credited him with £75 as a gesture of goodwill.”

• Jessica Gorst-Williams tackles consumer problems for Telegraph readers every week. To contact her, click here. If you want to ask a general money question, email moneyexpert@telegraph.co.uk. The best of the answers are included in our weekly newsletter